In recent years, particulate carbon substances, NOx, etc. in exhaust gas discharged from diesel engines or the like have gained notoriety as being harmful to environmental protection and health, and have thus been regulated. In order to remove the particulates to purify the exhaust gas, broadly-divided two methods as described below have been studied.
One is a method in which the particulates are captured by filtering exhaust gas using a metallic or ceramic filter having heat resistance and a three-dimensional porous construction, and if pressure loss is increased by this capture, the captured particulates are burned at a temperature of about 500 to 600° C. by using a burner, an electric heater, or the like to regenerate the filter.
The other is a method in which the particulates are self-burned by the action of a catalyst carried on a filter simultaneously with the particulate filtering operation.
In the case of the former method, pressure loss increases as the particulate removing effect is enhanced, so that the frequency of filter regeneration increases, which is thought to be economically disadvantageous. On the other hand, in the latter method, if there exists a catalyst that can keep its catalytic activity under the discharge conditions (gas composition and temperatures of exhaust gas from a diesel engine, power consumption can be reduced by lowering the heating temperature during filter regeneration. In addition, filter breakage caused by a high temperature can be prevented, so that this is thought to be an excellent method.
However, diesel engines use a heavy oil or light oil fuel usually containing sulfur, so that the exhaust gas contains much SO2. Also, depending on the diesel engine operating conditions, the oxygen concentration in the exhaust gas varies over a wide range of 5 to 15%. Under such exhaust gas conditions, an exhaust gas treatment filter regenerating method has not yet been established in which accumulated particulates are ignited and burned properly without the occurrence of secondary pollution.
On the other hand, as a method for removing NOx, there has been proposed a method in which hydrocarbon is introduced into the exhaust gas, and NOx in the exhaust gas is removed by reduction by using this hydrocarbon (for example, refer to Japanese Patent Publication No. 44-13002 (No. 13002/1969)).
Also, as a method in which no hydrocarbon is introduced into the exhaust gas, a method has been tried in which NOx is removed by reduction employing a particular catalyst using particulates and remaining hydrocarbon that are present in the exhaust gas as a reducing agent. Various studies have been carried out regarding catalysts for removing NOx. For example, Japanese Patent Provisional Publication No. 3-47539 (No. 47539/1991) discloses an exhaust gas treatment material formed by carrying a catalyst consisting of (a) an alkaline element, (b) a particular transition metal element, and (c) a rare earth element on a heat-resistant porous filter. This exhaust gas treatment material can efficiently burn and remove particulates contained in the exhaust gas, and can remove NOx and other harmful components. However, when the exhaust gas has a high temperature above 900° C., the catalyst has poor long-term heat resistance. Also, in exhaust gas treatment methods using other catalysts, it is difficult for NOx in exhaust gas that has a high oxygen concentration to be removed efficiently without introducing hydrocarbon. Thus, a method for effectively removing harmful components including NOx has not yet been established.
On the other hand, as an exhaust gas purifying catalyst for a moving or stationary engine, there has conventionally been known an exhaust gas purifying three-way catalyst which purifies exhaust gas by simultaneously effecting the oxidation of carbon monoxide (CO) and hydrocarbon (HC) and the reduction of nitrogen oxides (NOx) in the exhaust gas. As such a catalyst, a catalyst is generally used which is formed by applying silica or alumina slurry onto a fire-resistant carrier, for example cordierite, firing it, and carrying a noble metal such as Pd, Pt or Rh thereon. The performance of the exhaust gas purifying catalyst using such a noble metal is greatly influenced by the kind of engine and the preset air-fuel ratio. For a lean air-fuel mixture, that is, the lean side in which the air-fuel ratio is high, the quantity of oxygen in the exhaust gas after combustion is large, wherein the oxidizing action is active and reducing action inactive. Conversely, on the rich side in which the air-fuel ratio is low, the quantity of oxygen in the exhaust gas after combustion is small, wherein the oxidizing action is inactive and reducing action active. On the rich side, the removal of NOx using a catalyst is relatively easy, but on the lean side, the removal of NOx is difficult because of the inactive oxidizing action.
For a gasoline engine, at the time of ordinary running, operation is performed on the lean side during which combustion occurs with an air-fuel mixture of excess oxygen. wherever practicable due to the demand for lower fuel consumption. This means that a catalyst capable of removing NOx sufficiently even on the lean side is desired.
Also, exhaust gas discharged from a diesel engine is always under conditions on the lean side, so that it is difficult for NOx to be removed. Therefore, as in the case with a gasoline engine, a catalyst capable of removing NOx is desired. As an exhaust gas purifying catalyst in such an atmosphere with excess oxygen, there have been proposed various types of catalysts that simultaneously effect the oxidation of carbon monoxide (CO) and hydrocarbon (HC) and the reduction of nitrogen oxides (NOx). As one of these catalysts, for example, a Pt/Al2O3 catalyst in which platinum is carried on alumina has been proposed. However, even if the quantity of carried platinum is increased in the atmosphere with excess oxygen, it cannot be said that this catalyst has a practically sufficient NOx removal efficiency.
As a solution to this problem, there has been proposed an NOx removing method using a catalyst that utilizes frequent lean/rich changes in the range from the vicinity of stoichiometric air-fuel ratio to an over-lean air-fuel ratio as in the case of gasoline engine, or utilizes lean/rich changes obtained by the use of a method in which, for example, fuel is injected into exhaust gas, which method can be applied to all types of engines (Japanese Patent Provisional Publication No. 5-168860 (No. 168860/1993)). However, this NOx removing method has problems of high cost due to the use of noble metal and deteriorated performance due to sulfur (SOx) poisoning.
In Japanese Patent Provisional Publication No. 5-184928 (No. 184928/1993), a compound oxide catalyst having a perovskite structure represented by a general formula of Ln1−xRxMO3+a (0<x<1 and −1<a<0.2), which does not use a noble metal as a catalyst composition, has been proposed as an exhaust gas treatment material for effectively removing particulates and NOx in exhaust gas. Also, a method has been proposed which uses a catalyst formed by applying a perovskite compound oxide and by impregnating the applied compound oxide with a noble metal (Japanese Patent Provisional Publication No. 2001-269578). However, it cannot be said that either of these methods has sufficient NOx removal performance. Also, at present, like the Pt/Al2O3 catalyst, the performance is decreased by SOx, and a catalyst having excellent sulfur resistance has not been studied.